The field of the invention relates to systems and methods for determining physical properties of liquids, and particularly to methods which involve measuring the back pressure of a gas bubbling into a liquid.
Liquid exerts a hydrostatic head of pressure proportional to its density and depth. Bubbling a small flow of gas from a submerged dip tube causes gas back pressure which is equal to the liquid hydrostatic pressure. The gas pressure may be measured by a pressure transmitter to indicate certain characteristics or properties of the liquid. Systems employing single tubes have been frequently used to measure liquid levels. The bubble flow in such systems may be regulated to a very low level to increase the pressure in the tube until it balances the pressure at the end thereof. Pressure is then maintained at this level by the continuous release of air bubbles into the liquid.
Systems employing a pair of dip tubes have been used to measure the density of liquids and specific gravity. Such systems typically include two tubes of unequal length which are inserted within a liquid such that the end of one tube is at a different depth than the end of the other. Gas is flowed through the tubes and allowed to bubble through the liquid. The difference in pressure at the ends of the tubes is proportional to the density or specific gravity. Therefore, if a differential pressure transmitter is connected between the pair of tubes, which have a known vertical separation, the output signal can be used to calculate the liquid's density or specific gravity. U.S. Pat. Nos. 2,604,778, 3,460,394 and 4,419,893 disclose various systems which employ a pair of dip tubes through which a gas is bubbled in order to determine density or specific gravity.
Presently available and known apparatus using bubbling techniques to measure liquid level, density and/or specific gravity use a constant flow of air or other gas which is purged (bubbled) through the measured liquid from one or two vertically oriented tubes. Since pressure measurements are taken as the gas is bubbled through the liquid, the "noise" generated by the bubbles can lead to inaccurate pressure readings. Small inaccuracies are unacceptable when measuring the specific gravities of certain liquids such as the electrolyte for large lead acid storage batteries.